Work on methods and apparatus for the study of kidney and toad bladder epithelial cells grown as sheets on porous membranes continues to yield important results. The central element is the porous bottomed culture device (PBCD) in which the membrane is cemented to polycarbonate rings which provides electrical and chemical isolation between the solutions contacting each side of the membrane. With these PBCD's new knowledge of the development of epithelia is being obtained. For example, the development of vasopressin response in the cells (from African clawed toad kidney) requires that there be free exchange for materials between the cells and the media on the basal side (i.e. the PBCD's must spaced off the bottom of the dish). By this means, vasopressin response has been demonstrated for the first time in these cells. Work on PBCD's with transparent collagen membranes has been successful and is continuing. Medulary thick ascending limb cells from the rabbit kidney tubule have grown out from a short piece of tubule on 1 mm collagen membranes in about 10 days and produced apical side positive potentials as high as 23 millivolts.